Typically, magnetic fields are measured using flux gates or Hall probes (i.e., Hall effect switches). A Hall effect refers to the potential difference on the opposite sides of an electrical semiconductor through which an electrical current is flowing, created by a magnetic field applied perpendicular to the current. The applied magnetic field exerts a transverse force on the moving charge carriers in the flowing current, which tends to push the charge carrier to one side of the semiconductor. A buildup of charge at the sides of the semiconductor balances this magnetic influence, producing a measurable voltage between the two sides of the semiconductor. A measurement of the voltage created by the Hall effect can be used to obtain a measurement of the magnetic field. Because Hall probes use semiconductors to measure a magnetic field, the Hall effect method is subject to all the common limitations associated with semiconductors, such as sensitivity to temperature and changes over time to the semiconductor material. Further, Hall effect devices typically produce a very low signal level and thus require amplification.
Alternatively, changes in magnetic field strength can be measured using flux gates. A flux gate is made by winding coil around a core of an electromagnet (several turns of electric wire). In accordance with Faraday's Law, any change in the magnetic environment of a coil of wire will cause a voltage to be “induced” in the coil. Therefore, any changes of magnetic flux through the electromagnet core produce voltage on the flux gate, and the induced voltage is directly proportional to the time derivative of magnetic flux and the number of wire turns of the flux gate. Flux gates used to measure the strength of magnetic fields can be very sensitive to quick changes of magnetic field, and may also have a weak sensitivity to slow changes of magnetic field. Moreover, the flux gate measurement device must be periodically recalibrated with respect to a well-known magnetic field.